1. Field of the Invention
The present invention relates to a device for controlling the supercharging pressure of an exhaust gas turbocharger for an internal combustion engine in which the internal combustion engine is supplied with an intake gas for supercharging by a compressor which is rotated together with an exhaust gas turbine rotated by the energy of the exhaust gas from the internal combustion engine.
2. Description of the Prior Art
It is common that an internal combustion engine is equipped with an exhaust gas turbocharger to utilize the exhaust gas energy to supercharge intake air to the internal combustion engine, thereby increasing the intake air charging efficiency and therefore the amount of fuel injection, which greatly contributes to the increase of the engine output.
However, if the supercharging pressure becomes excessive, excessive stress is applied onto the internal combustion engine and onto its intake and exhaust gas system, thereby damaging them. Accordingly, many devices have been developed to prevent excessive increase in supercharging pressure.
An example of these devices is a velocity control device for exhaust gas flow as is described in U.S. Pat. No. 2,944,786. In this device, a control valve is provided in an exhaust gas intake port section of an exhaust gas turbocharger to adjust the degree of opening of the control valve, thereby controlling the velocity of the flow of the exhaust gas in the inlet port leading to the exhaust gas turbine so as to prevent an excessive rotation rate in the exhaust gas turbine. Consequently, the excessive supercharging pressure by the compressor coupled directly to the turbine is prevented while increasing the exhaust gas flow velocity in low load operation to increase the supercharging pressure in this operating state, thereby increasing the output.
Another example has a bypass channel provided to connect the upstream and downstream sides of the exhaust gas turbine, and the bypass channel is provided with a bypass control valve means which is adapted to open when the supercharging pressure in the bypass channel on the downstream side of the compressor reaches above a set value. When the supercharging pressure on the downstream side of the compressor exceeds a specified value, the bypass control valve means opens and the energy of the exhaust gas is exhausted to the outside through the bypass channel so that it does not act to rotate the exhaust gas turbine. This prevents excessive rotation of the exhaust gas turbine and consequently excessive increase in the supercharging pressure.
The velocity control device for exhaust gas flow and the bypass control valve device can effectively carry out supercharger pressure control, but the following inconveniences still remain.
Namely, in the velocity control device for exhaust gas flow, when the supercharging pressure exceeds the set value, the cross-sectional area of the exhaust gas inlet port is enlarged to decrease the flow velocity of the exhaust gas introduced into the exhaust gas turbine. But all the amount of the exhaust gas is introduced into the exhaust gas turbine and is not bypassed, so that when the exhaust gas energy is further increased, the rotation of the exhaust gas turbine cannot be controlled and the super-charging pressure is ultimately increased. Accordingly, the original structure design of the exhaust gas turbocharger must ensure that the increased amount of supercharging pressure is below an allowable value. For this reason, the overall torque characteristics must be set to generally produce low supercharging pressure throughout the entire engine operations including the low load operating of the engine. However, this results in that the supercharging pressure is reduced in the low load region as mentioned above, which is a problem area in the case of the exhaust gas turbocharger, thus making it difficult to improve the output characteristics in this operating region.
Now according to the bypass control valve means, when the supercharging pressure exceeds the set value, the exhaust gas is introduced into the bypass channel by the bypass control valve means, and is not used in the region of rotation of the exhaust gas turbine. It therefore functions as an effective means for preventing an excessive increase in the super-charging pressure. However, the torque characteristics determined by the structure of the exhaust gas turbocharger are constant, and simply act to prevent the excessive supercharging pressure, which does not solve the problems of the low super-charging pressure in the low load region. It should be further noted that with such a bypass control device, immediately after the bypass control valve is opened, the exhaust gas turbine rotates at an excessive speed due to the effect of its inertia, causing insufficient reduction of the supercharging pressure and generating knocking. The back pressure is rapidly lowered, giving rise to the fear that a catalytic device in the exhaust system could be damaged.
In addition, it was found out that when either of these devices is used separately and unoperated due to breakdown or seizure, the exhaust gas turbine and compressor rotate at an abnormally high speed and the supercharging pressure increases excessively, thereby generating knocking or engine damage.
Therefore, these devices as mentioned above must be further improved, and development is still required in the gas intake system for supercharging in order to maintain flow flexibility and pressure variability throughout the operation.